The present invention relates in general to micromachining, and in particular to a test apparatus for measuring friction or wear on a microscopic scale. The test apparatus can further be used as a microelectromechanical (MEM) fabrication process quality tool, and also as a test structure for assessing the reliability of MEM devices.
In microelectromechanical (MEM) devices, surface forces can play a relatively large role compared to gravity and inertia, which are the dominating forces on a macroscopic scale. High static friction is known to contribute to wear in MEM devices and can lead to device seizure, whereas kinetic friction (also termed dynamic friction) consumes a significant portion of the motive torque between rubbing or sliding members in a MEM device. Investigations into the failure modes of electrostatic microengines, for example, indicate that the usual path to failure involves adhesion between rubbing surfaces. Since surface forces are not well characterized and are often difficult to reproduce in MEM devices, most commercial MEM devices avoid any contact between structural members. If, however, sliding contact between the structural members of a MEM device is allowed, many more types of MEM devices can be fabricated, including shuffle motors and self-assembling structures (e.g. pop-up mirrors).
Relatively little is understood about friction on a microscopic scale in a MEM device when two surfaces come into contact. Thus, it is not currently understood how friction scales with apparent pressure, or with sliding velocity. Here, apparent pressure is that pressure which would be calculated for two surfaces coming into contact with each other when the two surfaces are assumed to be perfectly smooth. In a MEM devise, the actual pressure between these two surfaces can be substantially different when the roughness of the surfaces is taken into account. Thus, 2-10 nanometers of root-mean-square (rms) surface roughness which can be achieved in a MEM device can result in an actual pressure that is about one to two orders of magnitude larger than the apparent pressure would be for perfectly smooth surfaces.
In MEM devices, the apparent pressure and sliding velocity can vary by orders of magnitude depending upon a particular type of MEM device or on a range of operation of the device, with the apparent pressure generally varying in the range of about 0.1-100 megaPascal (MPa), and with the sliding velocity generally varying in the range of about 0.1-1000 microns per second (xcexcm-sxe2x88x921). Previous characterizations of friction on horizontal surfaces in MEM devices has been obtained using reciprocating comb-drive structures with contact between nonplanar surfaces. A disadvantage of these previous characterization methods is that the data could be obtained only over a limited range of pressure on the order of tens of kiloPascals due to a limited force which can be generated using a comb-drive structure. A further disadvantage of the previous method for measuring friction is that the comb-drive structure occupies a relatively large area on a semiconductor substrate, thereby limiting the area available for other MEM devices.
What is needed is a compact device for measuring friction on a microscopic scale over a relatively large range of pressure and velocity. Such a device could be incorporated onto a semiconductor substrate as a test structure to measure basic properties of friction that could then be used to model the behavior of other MEM devices fabricated on the same substrate.
An advantage of the present invention is that it provides a solution to a critical need to characterize the frictional properties of contacting surfaces for microelectromechanical devices and systems.
A further advantage of the present invention is that it provides an in situ test apparatus for determining friction and/or wear on contacting surfaces that have undergone a specified sequence of fabrication steps, thereby serving as a diagnostic process monitor.
Yet another advantage is that the friction test apparatus of the present invention occupies a relatively small substrate area compared to prior comb-drive structures, thereby allowing the apparatus of the present invention to be fabricated alongside a number of other MEM devices on a substrate for use in assessing the quality and reliability of the fabricated MEM devices.
Still a further advantage of the friction test apparatus of the present invention is that it allows friction measurements to be made over a much greater range of applied contacting pressure than is possible with prior devices, with the applied contacting pressure being independently controllable and variable from a horizontally-applied pressure used to generate a sliding motion of the apparatus.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to a friction test apparatus that comprises a pair of substantially planar contacting surfaces including a first surface formed above a substrate and a second surface formed on an elongate friction pad suspended above the substrate over the first surface and moveable into contact with the first surface for static or dynamic friction measurements. A rubbing motion of the first and second surfaces can be produced with the apparatus laterally along a longitudinal axis of the first surface. The friction test apparatus further comprises means for providing an adjustable vertically-directed force (i.e. a contacting force) to bring the second surface into contact with the first surface, and means for providing an adjustable horizontally-directed force to effect lateral movement of the second surface relative to the first surface.
In a preferred embodiment of the present invention, the friction test apparatus comprises a cantilevered driver beam supported at one end thereof above the substrate and substantially co-planar with the substrate, with an elongate friction pad suspended from the other end of the cantilevered beam. The end of the cantilevered beam upon which the friction pad is suspended can be forked with the friction pad attached to the forked end of the beam by one or more hinges. A bottom surface of the friction pad forms the second surface. Each hinge can be offset a distance from the end of the friction pad nearest the driver beam to maximize an area of contact between the first and second surfaces over a predetermined range of applied pressure. The contact pad containing the first surface can be formed on the substrate below the friction pad. In some embodiments of the present invention, more than one friction pad can be suspended from the cantilevered driver beam.
The means for providing the vertically-directed force can be a first electrostatic actuator comprising a pair of electrodes on each side of the friction pad to bring the friction pad into mechanical contact with the contact pad. In response to a first applied voltage, V1, a vertically-directed electrostatic force of attraction is generated between these electrodes to move the friction pad downward toward the substrate and into mechanical contact with the contact pad. The vertically-directed force is adjustable by controlling the magnitude of the first applied voltage, V1 which can be provided by a power supply or signal generator, with or without computer control.
The means for providing the horizontally-directed force can be the driver beam which preferably includes thereon an upper central electrode that is superposed above a lower central electrode located above the substrate to form a second electrostatic actuator for moving the friction pad laterally along the contact pad. These central electrodes are responsive to a second applied voltage, V2, to electrostatically bend the driver beam, shortening its horizontal extent, and thereby generating the horizontally-directed force to move the friction pad laterally along the contact pad. The horizontally-directed force can be adjusted by controlling the second applied voltage, V2 which can be provided by a power supply or signal generator, with or without computer control. A cyclic second applied voltage, V2, can be used to generate a rubbing motion between the first and second surfaces to determine dynamic friction between the surfaces, or to produce wear between the surfaces for assessing the reliability of contacting surfaces in other MEM devices formed on, the substrate.
The MEM friction test apparatus can further comprise means for determining a lateral displacement of the friction pad along the contact pad for use in calculating, in combination with the vertically- and horizontally-directed forces, a measure of friction between the friction pad and the contact pad. The means for determining the lateral displacement can comprise an optical interferometer, or a light beam (e.g. from a laser) that is directed onto an upper surface of the driver beam at an angle and reflected or bounced off the driver beam so that a spatial position of the reflected light beam can be sensed (e.g. by a position-sensing detector), or a pair of diffraction gratings, preferably with different grating periods, including a stationary grating on the substrate and a moveable grating on the driver beam above the stationary grating (i e. forming a Moirxc3xa9 interferometer).
The friction test apparatus can be used to measure friction between surfaces of different materials, with at least one of the friction pad and the contact pad generally comprising a material such as polycrystalline silicon (also termed polysilicon), silicon nitride, a dielectric (e.g. alumina), a metal (e.g. aluminum) or a metal alloy. The friction to be determined with the apparatus can be either static friction or dynamic friction (i.e. friction of motion), with the dynamic friction being measurable as a function of the velocity of movement of the friction pad laterally along the contact pad upon repeated actuation of the second electrostatic means with an alternating-current (ac) voltage, V2. Additionally, the apparatus can be used to measure a frictional loss resulting from a rubbing of the friction pad against the contact pad, or a force or energy of adhesion (also termed stiction) between the friction pad and the contact pad. Furthermore, the apparatus has applications for measuring wear within a MEM device, for assessing the reliability of one or more MEM devices fabricated on the same substrate as the friction test apparatus, or for use as a process quality tool during the fabrication of MEM devices.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.